Dual cycle turbine engine having increased efficiency and heat recovery system for use therein

ABSTRACT

A dual cycle engine which includes both a steam turbine and a gas turbine. The engine includes an air compressor and a combustion chamber connected to the air compressor for burning fuel with air from the air compressor to generate a first gaseous output which drives a gas turbine. The engine also includes a heat recovery system for recovering heat from the gaseous output of the gas turbine after the gaseous output has passed through the gas turbine. The heat recovery system includes a heat exchanger for extracting heat from the gaseous output and for heating an input water stream to generate steam. The steam is heated further in a superheater that burns fuel and air at the same pressure as the combustion chamber to generate a second gaseous output that is combined with the output of the combustion chamber prior to the output from the combustion chamber being passed through the gas turbine. The steam from the superheater is expanded in a steam turbine to generate work therefrom. In the preferred embodiment of the present invention, the heat exchanger is a supercritical heat exchanger. The water input stream is preferably generated by condensing water from the gaseous output after the gaseous output has passed through the heat exchanger. The superheater may be constructed by utilizing a heat exchanger in the combustion chamber. In embodiments of the present invention that utilize a steam injected gas turbine, the steam injection is provided by the output of the steam turbine.

This application is a Division of Ser. No. 08/710,159 filed Sep. 12,1996.

FIELD OF THE INVENTION

The present invention relates to gas turbine engines, and moreparticularly, to systems for increasing the efficiency of systemsutilizing a gas turbine engine.

BACKGROUND OF THE INVENTION

In a conventional single shaft gas turbine engine, a compressorintroduces air into a combustion chamber in which the air is mixed withthe burning fuel to produce gases that drive a turbine. The turbinedrives a load consisting of the compressor and an external load. In adual shaft gas turbine, the compressor is driven by a turbine that isseparate from the load turbine. The two turbines are not mechanicallyconnected. They are only gas dynamically connected. The gases from thefirst turbine pass through the second turbine after leaving the firstturbine. The compressor is usually driven by the high pressure turbinewith the combination of the compressor and turbine being referred to asthe gas generator. However, schemes in which the compressor is driven bythe low pressure turbine are also known. To simplify the followingdiscussion, a single shaft turbine will be used; however, it will beapparent to those skilled in the art that the teachings of the presentinvention may be equally applied to a dual turbine configuration. Theefficiency of such a turbine design improves with increasing operatingtemperatures; however, there is a limit to the operating temperaturedictated by the temperature at which the turbine blades and relatedsystems fail.

To further increase the efficiency of the engine, the energy that isdiscarded in the exhaust gases from the turbine must be reclaimed.Schemes in which the exhaust gases are used to heat water in a boiler togenerate steam for a steam turbine are known to the art. The efficiencyof the steam turbine is determined by the temperature of the steamwhich, in turn, is determined by the temperature of the exhaust gasesleaving the gas turbine. Since the exhaust gases are typically at atemperature of 1000° F., prior art systems utilize steam turbines thatoperate at temperatures of 1000° F. or less. Since the efficiency of thesteam turbine cycle is determined by the temperature of the input steam,any increase in the steam inlet temperature without changing the exhausttemperature will further improve the efficiency of the combined engine.

As noted above, to maintain the temperature below this maximumtemperature of the turbine blades, the fuel to air ratio in thecombustion chamber is maintained below the point at which stoichiometriccombustion of the fuel is achieved. The additional air maintains thegases below the maximum operating temperature. Unfortunately, the energyneeded to compress this additional air reduces the overall efficiency ofthe engine.

These observations have led to gas turbine designs in which steam and/orwater is injected into the combustion system. For example, Dah Yu Cheng(U.S. Pat. Nos. 3,978,661, 4,128,994 and 4,297,841) recognized thatsteam addition to the Brayton cycle can significantly increase the powerand efficiency of the engine provided heat is recovered from the exhaustgases. The power generated by the drive turbine at any given temperatureis determined by the specific heat of the gases expanding through theturbine. Since steam has about twice the specific heat of air, the useof steam as the coolant significantly improves the power that can begenerated by the turbine.

Unfortunately, the amount of heat that leaves the system in the exhaustgases also increases when steam is used. The exhaust gases generated ina steam injected engine leave at a higher temperature and have a higherspecific heat. Hence, in the absence of some form of heat recoverysystem, the overall efficiency of the engine decreases. Cheng used aheat recovery boiler to recover the heat from the exhaust gases of theturbine to produce steam. Because of the pinch point limitation on theoperating pressure of the heat recovery boiler, and hence the operatingpressure ratio of the turbine, the maximum achievable efficiency waslimited in this system. Patton and Shouman (U.S. Pat. No. 4,841,721)solved the pinch point problem by operating the combustor of the gasturbine at a pressure above the supercritical pressure of water. Theyreplaced the boiler by a series of regenerative feed water heaters.Shouman (U.S. Pat. No. 5,491,968) describes a combustion system composedof a wet oxidation reactor to which is added, in series, a second stagecombustor to produce the desired turbine inlet temperature. Thiscombustion system replaces the conventional gas turbine combustor when awet oxidation reactor is used.

Broadly, it is the object of the present invention to provide animproved heat recovery system for use in a gas turbine engine system.

It is a further object of the present invention to provide a heatrecovery system that improves the efficiency of water/steam injected gasturbine systems.

These and other objects of the present invention will become apparent tothose skilled in the art from the following detailed description of theinvention and the accompanying drawings.

SUMMARY OF THE INVENTION

The present invention is a dual cycle engine which includes both a steamturbine and a gas turbine. The engine includes an air compressor and acombustion chamber connected to the air compressor for burning fuel withair from the air compressor to generate a first gaseous output whichdrives a gas turbine. The engine also includes a heat recovery systemfor recovering heat from the gaseous output of the gas turbine after thegaseous output has passed through the gas turbine. The heat recoverysystem includes a heat exchanger for extracting heat from the gaseousoutput and for heating an input water stream to generate steam. Thesteam is heated further in a superheater that burns fuel and air at thesame pressure as the combustion chamber to generate a second gaseousoutput that is combined with the output of the combustion chamber priorto the output from the combustion chamber being passed through the gasturbine. The steam from the superheater is expanded in a steam turbineto generate work therefrom. In the preferred embodiment of the presentinvention, the heat exchanger is a supercritical heat exchanger. Thewater input stream is preferably generated by condensing water from thegaseous output after the gaseous output has passed through the heatexchanger. The superheater may be constructed by utilizing a heatexchanger in the combustion chamber. In embodiments of the presentinvention that utilize a steam injected gas turbine, the steam injectionis provided by the output of the steam turbine.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a conventional combined cycle gas turbineengine that has been retrofitted with a heat recovery system accordingto the present invention.

FIG. 2 is a block diagram of a combined cycle gas turbine engine havinga heat recovery system according to the present invention.

FIG. 3 is a block diagram of a power plant that utilizes a steaminjected gas turbine and a steam turbine according to the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention may be more easily understood with reference tothe modifications needed to a conventional gas turbine engine to providea heat recovery system according to the present invention. Refer now toFIG. 1 which is a block diagram of a conventional gas turbine engine 10that has been retrofitted with a heat recovery system according to thepresent invention. Combustion chamber 14 is supplied with air bycompressor 12. The fuel burned in combustion chamber 14 generatesadditional gases as well as heating the compressed air. The expandinggases drive turbine 16 which is connected to the load 18. Drive turbine16 also drives compressor 12 via shaft 17.

As noted above, there is a substantial amount of heat in the turbineexhaust which is wasted if the exhaust is vented to the atmosphere. Inaddition, the exhaust stream contains a substantial amount of water inthe form of superheated steam. In the present invention, the heat fromthe exhaust stream is captured in the supercritical heat exchanger 40.The use of a supercritical heat exchanger avoids the "pinch point"limitations encountered when conventional boilers are used to recapturethe heat from the exhaust stream. The gases leaving heat exchanger 40are exhausted to the atmosphere. The steam leaving the steam turbine iscondensed in condenser 30 which can operate at atmospheric orsub-atmospheric pressure. The operating pressure of the condenser mustbe chosen to produce the best economics of the engine in terms ofinitial cost as well as operating cost. The condenser is equipped with asump of the desired capacity. Additional water is supplied to thecondenser sump to compensate for any steam or water leakage from thesystem.

Condensed water is pumped by pump 36 to a pressure above the criticalpressure of water through heat exchanger 40. The heated supercriticalwater leaves heat exchanger 40 and is heated further in a secondcombustion chamber 60. Combustion chamber 60 is operated at the samepressure as combustion chamber 14. The exhaust gases from combustionchamber 60 are mixed with the gases from combustion chamber 14 and usedto drive turbine 16. Hence, the energy captured by combustion gases incombustion chamber 60 is utilized to drive the gas turbine.

The water/steam entering combustion chamber 60 is typically at atemperature of 1000° F. Combustion chamber 60 increases this temperatureto the same value as that at the turbine inlet which is about 2000° F.The steam is then applied to steam turbine 65 to drive load 68. Theadded temperature provided by combustion chamber 60 substantiallyincreases the efficiency of steam turbine 65 relative to theconventional steam turbine recovery systems. Since no waste heat isgenerated in combustion chamber 60, this increase in efficiency isobtained without any compensating loss in efficiency elsewhere in thesystem.

It should be noted that prior art dual cycle systems that utilizeboilers to capture the heat from the turbine exhaust stream cannotprovide the same level of efficiency as the present invention becausethe boilers are at the maximum pressure dictated by the pinch point. Togain the maximum increase in efficiency, such systems must use multipleboilers operated at different pressures. The additional boilerssubstantially increase the cost of such systems.

The embodiment of the present invention shown in FIG. 1 utilizes aseparate combustion chamber for generating the input to steam turbine65. While this embodiment is useful in understanding the presentinvention, the preferred embodiment of the present invention utilizes asingle combustion chamber which has been modified to include the heatexchanger for further heating the water from heat exchanger 40. Refernow to FIG. 2 which is a block diagram of a gas turbine engine having aheat recovery system according to the present invention. To simplify thefollowing discussion, those components that serve the same functions aslike components discussed with respect to FIG. 1 have been given thesame numerals as used in FIG. 1 for the components in question. Turbineassembly 100 differs from that shown in FIG. 1 in that combustionchambers 14 and 60 have been replaced by a single combustion chamber 160which includes a heat exchanger 161 for heating the water recapturedfrom the turbine exhaust stream that has been preheated in heatexchanger 40.

The above described embodiments of the present invention have utilized aconventional combined cycle gas turbine engine. As noted above,water/steam injected gas turbines provide significant advantages overconventional gas turbines. The present invention is particularly welladapted for use in water injected gas turbines. When the presentinvention is utilized with a conventional combined cycle gas turbine,the steam from the steam turbine is condensed in the condenser. Inconventional steam turbines, the exhausted steam is condensed to allowthe steam turbine to operate at the greatest possible temperaturedifferential. The cost of the low pressure equipment needed toaccomplish this is a substantial portion of the cost of a conventionalsteam turbine system. Alternatively, the condenser of the steam turbineoperated at a higher pressure and a bottom cycle as suggested by Pattonand Shouman (U.S. Pat. No. 4,841,721) may be used. However, once again,the additional equipment substantially increases the cost of the system.

The present invention avoids this costly low pressure equipment byutilizing the exhaust steam for the steam injection needed by the gasturbine. Refer now to FIG. 3 which is a block diagram of a power plant200 that utilizes a steam injected gas turbine comprising air compressor212, combustion chamber 260 and gas turbine 270. To simplify thefollowing discussion, those components that serve the same functions aslike components discussed with respect to FIG. 2 have been given thesame numerals as used in FIG. 2 for the components in question. Whilethe air compressor, combustion chamber, and gas turbine performanalogous functions to the components shown in FIG. 2, the capacitiesand characteristics of these components will, in general, differ fromthe capacities of the corresponding components, and hence, thesecomponents have been given different numerical designations.

Combustion chamber 260 includes a heat exchanger 161 and a steaminjection port 262. The steam source for port 262 is the exhaust steamfrom steam turbine 65. Hence, any heat remaining in the exhaust fromsteam turbine 265 is captured by combustion chamber 260 and used in theoperation of gas turbine 270. As a result, steam turbine 265 operateswith an exhaust steam partial pressure considerably lower thanatmospheric pressure even though the condenser operates at atmosphericpressure. This produces the same effect as condensing steam atsubatmospheric pressure without the need for a costly steam low pressureturbine section. The gases leaving heat exchanger 40 enter condenser 300which can operate at or below atmospheric pressure. The non-condensablegases are discharged through line 50 to the atmosphere. If the condenseris operated at sub-atmospheric pressure, the compressor must be used todischarge the gases through line 50. The condenser is equipped with asump of the desired capacity. A float valve maintains a constant waterlevel in the sump. Any excess water is drained through a pump when thecondenser is operated at sub-atmospheric pressure.

Pump 360 pumps the condensed water to a pressure above the criticalpressure of water through heat exchanger 40. The heated supercriticalwater leaves heat exchanger 40 and is heated further in combustionchamber 260 through the superheater 161. The superheated steam leavesthe superheater 161 and is fed to steam turbine 65. The steam leavingsteam turbine 65 is fed into the gas turbine combustion chamber 260where it is mixed with the combustion gases.

The steam and gas turbines have been shown driving different loads inthe embodiments discussed above. However, it will be apparent to thoseskilled in the art that these loads can be a common drive shaftconnected to a common load.

Various modifications to the present invention will become apparent tothose skilled in the art from the foregoing description and accompanyingdrawings. Accordingly, the present invention is to be limited solely bythe scope of the following claims.

What is claimed is:
 1. An engine comprising:an air compressor; acombustion chamber connected to said air compressor for burning fuelwith air from said air compressor to generate a first gaseous output; agas turbine driven by a second gaseous output passing therethrough; andheat recovery means for recovering heat from said second gaseous outputafter said second gaseous output has passed through said gas turbine,wherein said heat recovery means comprises: a first heat exchanger forextracting heat from said second gaseous output and for heating an inputwater stream to generate steam; a superheater for further heating saidsteam, said superheater burning fuel and air at the same pressure assaid combustion chamber from said air compressor to generate a thirdgaseous output that is combined with said first gaseous output to formsaid second gaseous output; and a steam turbine for expanding said steamto generate work therefrom and creating a steam output.
 2. The engine ofclaim 1 wherein said first heat exchanger comprises a heat exchanger inwhich water is maintained at a pressure above the critical pressure ofwater.
 3. The engine of claim 1 further comprising means for condensingwater from said steam output, said condensed water providing at least aportion of said input water stream.
 4. The engine of claim 1 whereinsaid combustion chamber further comprises means for receiving steam andcombining said steam with gases generated by the burning of said fuel,said steam/gas mixture comprising said first gaseous output.
 5. Theengine of claim 1 further comprising means for condensing water fromsaid first gaseous output after said gaseous output has passed throughsaid heat exchanger, said condensed water providing said input waterstream.
 6. A heat recovery system for generating work from exhaust gasesof a gas turbine engine, said gas turbine engine being driven by theexpansion of a first gaseous output comprising second and third gaseousoutputs, said second gaseous output being generated by a combustionchamber in which fuel and air are burned, said heat recovery systemcomprising:a first heat exchanger for extracting heat from said exhaustgases and for heating an input water stream to generate steam; asuperheater for further heating said steam, said superheater burningfuel and air at the same pressure as said combustion chamber to generatesaid third gaseous output; and a steam turbine for expanding said steamto generate work therefrom and creating a steam output.
 7. The heatrecovery system of claim 6 wherein said first heat exchanger comprises aheat exchanger in which water is maintained at a pressure above thecritical pressure of water.
 8. The heat recovery system of claim 6further comprising means for condensing water from said exhaust gasesafter said exhaust gases have passed through said first heat exchanger,said condensed water providing said input water stream.
 9. The heatrecovery system of claim 6 further comprising means for condensing waterfrom said steam output, said condensed water providing at least aportion of said input water stream.